Using Autumn Hawk Watch to Track Raptor Migration and to Monitor Populations of North American Birds of Prey1
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Using Autumn Hawk Watch to Track Raptor Migration and to Monitor Populations of North American Birds of Prey1 Kyle McCarty2 and Keith L. Bildstein2 ________________________________________ Abstract Raptors are secretive, area-sensitive predators whose (Bildstein 1998). One particularly cost-effective populations can be logistically difficult and financially method for monitoring populations of these birds is to prohibitive to monitor. Many North American popula- sample regional and even continental populations at tions of raptors are migratory however, and on mig- traditional migratory bottlenecks and concentration ration raptors are frequently counted at traditional points (Zalles and Bildstein 2000). An incipient net- migration watchsites. Experiences at Hawk Mountain work of raptor migration watchsites (i.e., lookout Sanctuary (HMS) and elsewhere suggest that long-term points typically situated on mountain-tops, coastal migration counts can be used to monitor regional plains, river valleys, and lakesides and other migration populations of raptors. Hawkwatchers have collected “leading lines”) exists in North America, and during count data on standardized Hawk Migration of North the past 70 years migrants have been counted on a America (HMANA) Daily Report Forms since the mid- regular or irregular basis at more than 1,800 short-term 1970s. In 1998 HMS, HMANA, the National Audubon or permanent watchsites (McCarty et al. 2000). Society, and the Lab of Ornithology at Cornell Uni- versity began Internet-based data entry of migration Although most raptor migration watchsites are in the watchsite counts at the BirdSource website. By autumn northeastern United States, the recent addition of 2002, the Autumn Hawk Watch web page was col- season-long migration counts at sites in Gulf Coast lecting and displaying daily count reports from 66 Texas (Smith et al 2001) and Mexico (Zalles and watchsites in three Canadian provinces, 26 United Bildstein 2000), and in Caribbean slope Costa Rica States, and in Mexico, Costa Rica, and Bolivia. Autumn (Bildstein, pers. obs.), together with the establishment Hawk Watch provides participants and interested of an array of watchsites in the western United States parties with near real-time maps and tables that in the 1980s (Smith and Hoffman 2000), has created a document the movements of raptor migration across potentially effective network for assessing regional and the Americas each autumn. The web page also captures continental populations of migratory birds of prey. count data for later use in monitoring raptor pop- Since the mid 1970s, most migration data have been ulations, and provides HMANA with timely summaries collected using Hawk Migration Association of North of each count, which are published in the HMANA America (HMANA) Daily Report Forms, or “green Journal of Hawk Migration Studies. sheets.” As such, much of the information collected at these sites is compatible and regional and continental assessments of population change are possible. Unfor- tunately, most of this information is not available in Key Words: Autumn Hawk Watch; Hawk Migration electronic format in a single database, making broad- Association of North America; Hawk Mountain scale, multi-site analyses difficult. However, monitor- Sanctuary; Internet; migration monitoring; raptors. ing numbers of migrating hawks at single migration watchsites has been used to great effect (e.g., Carson 1962, Newton 1979, Kerlinger 1989, Bednarz et al 1990, Zalles and Bildstein 2000, Mueller et al. 2001), and the use of data from networks of sites holds great Introduction promise. The establishment of several North American- style, season-long count efforts in Mesoamerica and Raptors are wide-ranging and often secretive predatory the Caribbean Basin in the past 10 years, for example, birds, many of whose populations are logistically dif- increases the potential for tracking world populations ficult and financially prohibitive to survey and monitor of three species, Mississippi Kite (Ictinia mississip- __________ piensis), Broad-winged Hawk (Buteo platypterus), and Swainson’s Hawk (B. swainsoni), as well as conti- 1 A version of this paper was presented at the Third Interna- nental populations of a fourth species, Swallow-tailed tional Partners in Flight Conference, March 20-24, 2002, Kite (Elanoides forficatus) (Zalles and Bildstein 2000, Asilomar Conference Grounds, California. 2Hawk Mountain Sanctuary Acopian Center, 410 Summer Bildstein and Zalles 2001). Valley Road, Orwigsburg, Pennsylvania 17961. E-mail: [email protected]. USDA Forest Service Gen. Tech. Rep. PSW-GTR-191. 2005 718 Autumn Hawk Watch - McCarty and Bildstein Here we (1) report several examples of how a single- States in the early 1970s. Even more remarkable is that site, long-term migration watchsite database has been shifts in overall numbers of Bald Eagles seen at the site used to track changes in raptor populations and mig- lagged declines and subsequent increases in the ratios ration behavior, and (2) describe a new webpage that of juvenile-to-adult birds by 5 to 10 years, exactly as uses the power of the Internet to capture, analyze, and expected if the shifts in ratios of age classes reflected display the results of counts at a network of migration shifts in reproductive success in the monitored pop- watchsites across North and Central America. ulation (Bildstein 1998). A more recent example of the utility of the HMS database involves its use, together with counts from The Conservation Potential of other migration watchsites and information from add- Migration Watchsite Count Data itional geographically explicit databases. Beginning in the 1980s and extending into the early 1990s, many Hawk Mountain Sanctuary (HMS) was founded in migration watchsites in the northeastern United States 1934 by conservationist Rosalie Edge to stop the began reporting substantial declines in numbers of shooting of thousands of migrating raptors along the Sharp-shinned Hawks (Accipiter striatus). The declines Kittatinny Ridge in the central Appalachian Mountains were especially notable at coastal watchsites, where of eastern Pennsylvania, 120 km northwest of Phila- flights consisted largely of juvenile individuals. Al- delphia, Pennsylvania. In late September 1934, Hawk though initial reports of the decline were accompanied Mountain’s Maurice Broun began counting migrating by suggestions of natural population cycling or shifts in raptors from what was then called Observation Rocks. wintering range, by the early 1990s additional reports Although Broun’s counts were initiated primarily to became more ominous, and included suggested links to document the numbers of raptors being “saved” at the widespread habitat or prey-base loss induced by acid Sanctuary, so as to enlist financial support for the precipitation or declines in populations of Neotropical conservation effort, it quickly became apparent that a songbird migrants (Viverette et al. 1996). series of annual counts would enable conservationists to monitor regional populations of birds of prey. To- To test the hypothesis that the declines resulted from day, Hawk Mountain maintains the longest and most northward shifts in wintering areas of the regional complete record of raptor migration in the world population, researchers at Hawk Mountain compared (Bildstein and Compton 2000). migration count data from the Sanctuary and Cape May Point, New Jersey with those from National Audubon One of the first conservation uses of the Hawk Moun- Society Christmas Bird Counts (CBCs) north and south tain long-term database was an analysis of annual of the two migration watchsites. The analysis of CBCs ratios of juvenile and adult Bald Eagles (Haliaeetus conducted from 1979 through 1989 revealed that num- leucocephalus) seen at Hawk Mountain before and th bers of Sharp-shinned Hawks seen on CBCs north of after the widespread use of DDT in mid-20 Century the two migration count sites increased substantially North America. A bimodal pattern in the seasonal whereas numbers seen on CBCs south of the two timing of the flight of Bald Eagles at Hawk Mountain, watchsites had not. The results support the shift-in- with a major movement in late August-early Septem- migration behavior hypothesis (i.e., migratory short- ber, and a second smaller movement peaking in mid- stopping). That Cape May Point counts declined more November, suggested that although both “southern” abruptly than those at Hawk Mountain may reflect the and “northern” birds migrated at the site, most of the fact that the Sharp-shinned Hawk flight at the former birds were southern Bald Eagles nesting in Florida. In site consists almost entirely of juvenile individuals, 1952, a concerned Maurice Broun began commenting whereas that at the latter includes many more adults. on a substantial decline in the ratio of juvenile-to-adult Juveniles are more likely to modify their migration Bald Eagles at the site, which began in the late 1940s patterns in light of changed environmental conditions and, thereafter, remained low through the 1950s, than are adults (Berthold 1993). 1960s, and mid-1970s (Bildstein 1998). A decade after Broun reported the shift, Rachel Carson used the same Although the reason for the shift in behavior remains database in Silent Spring to support her arguments for uncertain, the change coincided with a series of part- the impact of organochlorine pesticides on populations icularly mild winters, as well as with increases in the of Bald Eagles and other species of predatory